In general, cylindrical batteries are configured such that a power-generating element is accommodated in a bottomed metal battery case, with its opening sealed with a metal sealing plate. In secondary batteries, such as lithium ion secondary batteries, the power-generating element is comprised of an electrode group and an electrolyte. The electrode group is formed by spirally winding a positive electrode and a negative electrode, with a separator sandwiched therebetween. The separator insulates the positive electrode and the negative electrode from each other, and has a function of holding the electrolyte.
The sealing plate has a valve mechanism configured to ensure battery safety. In the event where an abnormal condition occurs in the battery, and the pressure in the battery case increases to a predetermined value, the valve mechanism is opened to release the gas in the battery case, thereby preventing an accident, such as a crack in the battery case.
Recently, however, with increasing functionality of electric devices, further increases in battery capacities are promoted, which as a result further increases a pressure in the battery case in the event where an abnormal condition occurs in the battery. In particular, the upper insulating plate on the edge of the electrode group may not be able to withstand the pressure and be deformed, and the electrode group may move upward and close an exhaust hole. To avoid this, various techniques for ensuring battery safety have been suggested.
For example, Patent Document 1 discloses a technique in which a diameter of the upper insulating plate is equal to or larger than an inside diameter of a groove formed in the battery case to reduce displacement of the electrode group.